An imaging device mounted in a digital camera, video camera, or the like includes a CMOS (Complementary Metal Oxide Semiconductor) image sensor, or a CCD (Charge Coupled Device) image sensor, which is formed on a semiconductor substrate. A defective pixel may occur in such an imaging device due to a local crystal defect on the semiconductor substrate.
Such a defective pixel ends up outputting a specific pixel value that is not dependent on the amount of incident light. Therefore, if the defective pixel exists in the imaging device, a captured image including an error pixel value is output so that the image quality deteriorates.
Many of the known imaging apparatuses such as video cameras and digital cameras have the following countermeasure configuration to prevent an error based on such a defective pixel. In other words, address data indicating the location of a defective pixel in a solid-state imaging device is prestored in a memory device such as nonvolatile memory in the manufacturing stage. A signal output from the defective pixel is corrected and output based on the address data indicating the defective pixel stored in the memory device upon image capture. This is the countermeasure.
For example, in a case where a CMOS image sensor is mounted in an imaging apparatus, an output signal of each pixel of the CMOS image sensor and output signals of neighboring pixels of the pixel to be examined are compared in the manufacturing stage in a state where a lens of the imaging apparatus is shielded from light and light does not enter the CMOS image sensor. If the difference between these output signal values exceeds a predetermined threshold value, the pixel that has output this specific signal is determined to be a defective pixel.
Furthermore, address data indicating the location of the pixel determined to be a defective pixel is stored in nonvolatile memory. When the defective pixel detection ends, the CMOS image sensor and the nonvolatile memory in which the address data of the defective pixel is stored are integrated in the imaging apparatus for shipment.
When a user captures an image using the imaging apparatus, an output signal of the defective pixel included in an output video signal from the CMOS image sensor is corrected, using output signals of the pixels in the vicinity of the detective pixel, based on the address data of the defective pixel of the CMOS image sensor stored in the nonvolatile memory. An image having the corrected pixel value signal is then output as an output image.
Such a method is used as the known countermeasure against defective pixels in many cases.
However, the process is effective strictly and only for a defective pixel found in the manufacturing stage of the imaging device. There is a problem that the above countermeasure cannot deal with a defective pixel of the imaging device occurring due to aging after the shipment of the imaging apparatus.
In order to deal with such a problem, for example, Patent Document 1 (JP 06-6685 A) discloses a defect correction apparatus that, upon the turning-on of the power to an imaging apparatus, closes an aperture of a lens mounted in the imaging apparatus to enter a light-shielding state, detects a defective pixel with an imaging output signal of a solid-state imaging device, records and holds defect data based on a detection signal from the defective pixel, and corrects the defective pixel, using the latest defect data upon image capture.
However, such a defect correction configuration needs to store address data of the defective pixel, and the like in memory, and memory capacity therefor needs to be secured. Accordingly, there is a problem to result in an increase in the cost of the apparatus. Moreover, there is also a problem that the number of correctable defective pixels tends to depend on the storage capacity of the memory in which the address data of the defective pixels is stored.
Furthermore, for example, Patent Document 2 (JP 2008-154276 A) and Patent Document 3 (JP 2009-105872 A) disclose configurations that also enable the detection and correction of a defective pixel that occurs after shipment without mounting a memory in which the locations of defective pixels are stored.
Patent Document 2 discloses the configuration that verifies, for example, a texture direction around a pixel of interest selected from a captured image and detects and corrects a defect with reference to the texture direction. The detection accuracy and the correction accuracy are improved by the process with reference to the texture direction.
However, there is a problem with the configuration that the performance in the detection and correction of a defect is decreased in an image area where the texture direction around a pixel of interest cannot be obtained accurately, such as a flat area. Moreover, there is also a problem that the calculation cost and the circuit size are increased to determine the texture direction accurately.
Moreover, Patent Document 3 discloses the configuration that uses the standard deviation value of neighboring pixels for the detection of a defective pixel based on a captured image. However, the value of the standard deviation is also increased at an edge of the image. Accordingly, there is a problem that such a detection error as that determines the edge area to be a defective pixel occurs, and a wrong pixel value correction is made to contrarily deteriorate the image quality.